Mikaeus, the Unhallowed + Spawning Pit + Slumbering Walker

Mikaeus, the Unhallowed

Spawning Pit

Slumbering Walker

How it works

1. Activate Spawning Pit by sacrificing Slumbering Walker.
2. When Slumbering Walker dies, its undying ability triggers, returning it from your graveyard to the battlefield without a +1/+1 counter on it due to its replacement effect.
3. Resolve the Spawning Pit ability
4. Repeat.